Bow-molded hoses and apparatuses for producing the same

ABSTRACT

The blow-molded hose of a synthetic resin of the present invention is produced by blow-molding a single parison comprising a soft resin and a hard resin. The blow-molded hose comprises a monolayer (or single-layer) segment formed with either a soft resin layer or a hard resin layer and multilayer laminating) segment formed with the soft resin layer and the hard resin layer, wherein, in the multilayer (laminating) segment, the thickness of the resin layer forming the monolayer segment is thinner than that of the other resin layer. Practically, the monolayer segment comprises the soft resin layer and forms the connection end(s) at the end(s) of the hose.

This application is a division application of Ser. No. 09/025,340, filedon Feb. 18, 1998, now U.S. Pat. No. 6,214,268.

FIELD OF THE INVENTION

The present invention relates to a blow-molded hose of a synthetic resinuseful for use, for example, ducts of an intake system of an automobileengine, and to an apparatus for producing the blow-molded hose.

BACKGROUND OF THE INVENTION

As the blow-molded hose of the synthetic resin, Japanese Patent Laidopen No. 14445/1996 (JP-A-8-14445) discloses a blow-molding pipe orhose. The blow-molded hose is prepared by blow-molding a parison havinga laminating structure of a soft resin layer and a hard resin layer withvarying partially the thickness ratio of the soft and hard resin layersin the longitudinal direction in order to form mutually a flexible softportion and a rigid portion for shape reinforcement. However, when theflexible soft portion is formed at a joint or connection edge of theduct or hose for connecting with other parts or members, the flexibilityof the soft resin portion is deteriorated by the hard resin since thehard resin layer is laminated on the soft resin layer irrespective ofthe thickness ratio. Therefore, the joint or connection characteristicswith the other members such as a metal pipe are lowered and the highsealing at the connection site cannot be realized.

A multilayer (multi-walled) hose or tube is manufactured by extrudingmolten resin in the form of a parison from an annular die slit, settingthe parison in a mold, and carrying out blowing. In this moldingoperation, molten resin from a resin feeding side-port flows down acircumferential annular passageway and is extruded in an axial(downward) direction from an annular die slit. Therefore, not only theflow and flow rate of molten resin are complicated but also a largepressure disparity between the part corresponding to the resin feedingport and the part opposed to the feeding port, as well as a pressurestrain, is created to cause variations in thickness, with the resultthat a hollow body of uniform wall thickness can hardly be obtained.Particularly in the extrusion-blow molding of a multi-walled hollowproduct, such a variation tends to occur more readily in eachconstituent layer.

To correct for such variations in thickness of the parison and hollowproduct, it is common practice to displace the die in a horizontaldirection with an adjusting bolt so as to change the slit dimensionbetween the die and the mandrel. Japanese Patent Laid-open No.(JP-A-)51-134758 discloses an extrusion molding technology for theproduction of a multi-walled hollow tube wherein while the thicknessratio of constituent layers of the multi-walled structure is controlledby judicious setting of the co-extrusion speeds of the ring piston, theexit width of the resin passageway downstream of the point ofconvergence of resin streams for respective layers is adjusted by thevertical displacement of the mandrel or the horizontal displacement ofthe die so as to control the wall thickness. This literature furtherteaches the utilization of the horizontal displacement of the die foroffset control.

Japanese Patent Publication No. (JP-B-)3-5286 discloses a prior arttechnology which comprises constituting a die with a flexible ring andpressing both lateral sides of the die with actuators to deform the dieinto an elliptical shape to thereby adjust the annular slit between thedie and the mandrel and correct for a variation in thickness of theparison.

However, since the points of variation in thickness of a parison are notdiametrically opposed, it is still difficult to obtain a parison andhollow tube of uniform wall thickness even if the thicknesses at opposedpoints are adjusted. Particularly in the production of a multi-walledhollow product, thickness variations may occur in respective constituentlayers so that the thickness of the parison cannot be controlled withaccuracy.

Furthermore, in the intermittent production of parisons and hollow tubesusing an extrusion-blow molding machine, the variations in thicknessresult in resin leaks through the clearance between the die and themandrel. Thus, the intermittent production of parisons and hollow tubescomprises raising the ring piston to close the annular slit between thedie and the mandrel, collecting a parison-equivalent amount of moltenresin in the accumulation chamber, driving a rod of the mandrel forwardto form an annular slit, lowering the ring piston to expel the moltenresin from the accumulation chamber to form a parison having a wallthickness corresponding to the slit dimension, and driving the rod andmandrel back to close the annular slit, all as a cycle on anintermittent basis.

Such being the process, when the rod is driven back after the abovethickness adjustment of the parison by the adjusting bolt, the annularslit cannot be hermetically closed by the mandrel because of the offsetof the die with respect from the mandrel so that a gap typically as wideas about 1 mm is formed between the die and the mandrel. Then, as moltenresin is collected in the accumulation chamber with the annular slithaving been closed by the mandrel, the resin leaks out from the gap.Therefore, in the long-time repeated production of many parisons andhollow products, a large waste of resin takes place to detract fromeffective utilization of the resin and economics of production.

Furthermore, as the rod is driven back with a strong force with the dieoffset from the mandrel, a bending force, instead of an axial force,acts on the rod along the taper of the die and the mandrel to bend therod and injure the joint between the mandrel and rod. Therefore, the rodshould be driven back as gently as possible to let the mandrel beabutted against the head.

Moreover, according to the above operation, a monolayered flexiblesegment or rigid segment and a laminated segment of the hose or tube canhardly be constructed with high precision because the resin leakagecannot be completely prevented.

Japanese Patent Publication No. (JP-B-)52-37026 discloses an extrusionmolding apparatus for multi-walled tubes which, for the purpose ofpreventing variation in thickness and bending of the parison withoutadjusting the clearance between the die at the tip of a head and themandrel, comprises a cylindrical wall element having a temperatureregulating function disposed in a laminar arrangement to provide aplurality of cylindrical resin accumulation chambers within ablow-molding head, an extruder for extruding a resin into theaccumulation chambers, and a slidable ring piston for expelling theresin from the accumulation chambers. Japanese Patent Publication No.(JP-B-)57-53175 discloses an extrusion molding apparatus comprising amolding head having a die and an annular passageway, a cylindricalpartitioning wall and a ring piston as disposed in a laminar, verticallyslidable relation in the molding head so as to form a cylindricalaccumulation chamber, and an extruding means for extruding a polymerfrom the cylindrical accumulation chamber. It is taught in theseliterature that the ring piston is raised (driven back) to preventleakage of resin from the clearance between the die and the mandrel.

However, since the resin is aspirated into the accumulation chamber bydriving the ring piston back so as to prevent leakage, it is necessarythat an excess or more than one parison-equivalent of resin should beaccumulated in the accumulation chamber. Therefore, the excess of resinin the accumulation chamber is also extruded in the formation of aparison, so that the desired effective utilization of resin cannot bematerialized.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a blow-molded hoseor tube comprising a flexible segment and a rigid segment, wherein thesegments are precisely or accurately formed in the longitudinaldirection, and to provide an extrusion molding apparatus for producingthe blow-molded hose or tube.

It is another object of the present invention to provide a blow-moldedhose or tube, an extrusion molding apparatus and a hollow tube moldingmethod, wherein the high sealing properties at the connection or jointedges or end(s) of a hose can be achieved.

It is a further object of the present invention to provide ablow-molding hose or tube, an extrusion molding apparatus and a hollowtube molding method, wherein the soft and hard segments of a hose can beaccurately constructed with closing tight the slit between the die andthe mandrel to preclude resin leaks with certainty.

The inventor of the present invention did much research to accomplishthe above-mentioned objects and found that if a single-layered segment(or unit) of a soft resin or a hard (or rigid) resin and a multilayeredsegment (or unit) having a laminating structure comprising a soft resinlayer and a hard resin layer are formed mutually in the longitudinaldirection, a high flexibility and shape reinforcement can be imparted tothe hose by a flexible segment and a rigid segment. The presentinvention has been developed on the basis of the above finding.

Thus, the blow-molded hose of a synthetic resin according to the presentinvention is a hose produced by blow-molding a single parison,comprising a soft resin and a hard resin, and which comprises amonolayer (or single-layer) segment formed with either a soft resinlayer or a hard resin layer and a multilayer (laminating) segment formedwith the soft resin layer and the hard resin layer, wherein, in themultilayer (laminating) segment, the thickness of the resin layerforming the monolayer segment is thinner than that of the other resinlayer of the two resin layers. Practically, the monolayer segmentcomprises the soft resin layer and forms the connection end(s) at theend(s) of the hose. Since the hose or tube of the present inventionprovides with the monolayer segment of, for example, the soft resinlayer, the other resin layer such as the hard resin layer does notadversely affect on the properties of the soft resin layer. Thus, abellow portion of the hose and/or the end of the hose may be formed withthe soft resin as the monolayer segment, the high flexibility can beimparted to the bellow portion and/or the connection end of the hose.

Further, the present hose is manufactured with a parison, the moldingmay be conducted with use of a conventional multi-blow moldingapparatus, and it is unnecessary to use a molding apparatus providedwith a complicated mechanism such as a completely resin-alternatingmechanism (for example, a shut/off valve) which is disclosed in JapanesePatent Laid open No. 245912/1993 (JP-A-5-245912) and which is requiredfor formation of a single-layer parison.

The extrusion molding apparatus of the present invention comprises, forextruding a parison corresponding to the hose from an annular slit on anintermitted basis, a die, a mandrel which is axially slidable withrespect to the die to form an annular slit therebetween, a rod fordriving the mandrel axially with respect to the die, an annularpassageway including a molten resin accumulation chamber communicablewith the annular slit, a supply means for feeding the molten resin tothe annular passageway. The apparatus further comprises a thicknessadjusting means for displacing the die from an original positionadjoining to the mandrel in timed relation with the advance of a rod toadjust the wall thickness of a parison being extruded from the annularslit, an extruding means for extruding the resin accumulated in themolten resin accumulation chamber through the annular slit to form aparison, a resetting means which, in response to completion of extrusionof the resin, resets the die to the original position, and a rod drivingmechanism for driving the rod forward to form the annular slit and, inresponse to resetting of the die, driving the rod in a reverse directionto close the annular slit, wherein the annular passageway comprises afirst annular passageway, for a first resin either of a soft resin or ahard resin, including a first molten resin accumulation chambercommunicable with the annular slit and a second annular passageway, fora second resin of the other resin of the two resins, including a secondmolten resin accumulation chamber communicable with the annular slit andbeing joinable the second resin with the first resin, the supply meanscomprises a first supply means for feeding the first molten resin to thefirst annular passageway and a second supply means for feeding thesecond molten resin to the second annular passageway, and the extrudingmeans comprises a first extruding means for extruding continuously thefirst resin accumulated in the first molten resin accumulation chamberand a second extruding means for extruding, at a predetermined timeinterval, the second resin accumulated in the second molten resinaccumulation chamber.

The molding method of the present invention comprises extruding a resinaccumulated in a molten resin accumulation chamber from an annular slitdefined by a die and a mandrel to from a parison, wherein the methodcomprises a step of driving the rod forward to open the annular slit anddisplacing the die from an original position adjoining to the mandrel tocorrect for the thickness of the parison to be extruded from the annularslit, a step of extruding continuously the first resin accumulated inthe first accumulation chamber from the annular slit, with extruding andjoining, at a time interval, the second resin accumulated in a secondaccumulation chamber with the first resin, a step of resetting the dieto the original position after extrusion of resin, a step of driving therod back after the resetting of the die to cause the annular slit to beclosed by the mandrel, and a step of accumulating molten resins in theaccumulation chambers after closure of the annular slit, all of thesteps being executed as a cycle to form parisons on an intermittedbasis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generally section view showing an exemplary blow-molded hoseof the invention.

FIG. 2 is a generally longitudinal section view showing an exemplaryextrusion molding machine of the invention.

FIG. 3 is a general plan view showing an exemplary section adjustingmeans shown in FIG. 2.

FIG. 4 is a longitudinal section view of the section adjusting meansshown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is now described in detail with reference to theaccompanying drawings.

Now referring to FIG. 1, a blow-molded hose 1 of a synthetic resinaccording to the present invention comprises a monolayer segment (unit)3 of a synthetic soft resin layer 2 and a multilayer (laminating)segment (unit) 5 in which a synthetic hard resin layer 4 and thesynthetic soft resin layer 2 are laminated each other, and is producedby blow-molding a single parison corresponding to the hose 1. Themonolayer segment 3 and the multilayer (laminating) segment are formedintegrally and arranged mutually in the longitudinal direction. Thesynthetic soft resin includes, for example, olefinic resins such aspolyethylens (a low-density polyethylene, a middle-density polyethylene,a linear low-density polyethylene and etc.), ethylene copolymers(ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer andetc.), a plastisized polyvinyl chloride, polyurethanes, thermoplasticelastomer(polyester elastomer, polyamide elastomer, polyurethaneelastomer, olefinic elastomer and etc.) and so on. The examples of thehard resin are olefinic polymers (polypropylens, a high-densitypolyethylene, ethylene-propylene copolymer and the like), a polyvinylchloride, and the like.

Further, in the multilayer (laminating) segment 5, the thickness of thesynthetic hard resin layer 4 is thicker than that of the synthetic softresin layer 2. Namely, the soft resin layer 2 forming the monolayersegment 3 is thinner than that of the hard resin layer 4.

In the above embodiment, the monolayer segment portion may be molded andformed in the form of a bellow structure or formed as a connection orjoint site at the edge(s) or end(s) of the hose. Further, the monolayersegment may be formed with the synthetic hard resin, and, in themultilayer segment, the thickness ratio of the soft resin layer relativeto the hard resin layer may become larger.

The thickness ratio of the soft resin layer relative to the hard resinlayer in the multilayer segment may be, for example, about 5/95 to95/5(%), preferably 10/90 to 90/10(%), and more preferably 20/80 to80/20(%).

FIG. 2 is a generally longitudinal section view showing an exemplaryextrusion molding machine of the invention, FIG. 3 is a general planview showing an exemplary section adjusting means shown in FIG. 2, andFIG. 4 is a longitudinal section view of the section-adjusting meansshown in FIG. 2. The illustrated apparatus is intended for themanufacture of hollow tubes comprising a monolayer segment of a softresin and a multilayer (three-walled) segment comprising an inner softresin layer, an intermittent intermediate hard resin layer and an outercovering layer.

This extrusion molding apparatus comprises a head 11 equipped with a die12 at its lower end, a rod 13 disposed slidably in an axial (vertical)direction in the axial space within the head 11, a mandrel 14 secured tothe forward end of the rod 13 and adapted to form an annular slit 15with a lip portion of the die 12 in association with the axialadvancement of the rod 13 with respect to the die 12, and a plurality of(3 in the illustrated embodiment) annular passageways 16, 17, and 18formed in concentric relation within the head 11. The mandrel 14 isslidable together with the rod 13 in response to the operation of acylinder such as a hydraulic cylinder.

Of the above-mentioned annular passageways 16, 17, and 18, the firstpassageway (innermost annular passage way) 16, for the first resin suchas the soft resin, that forms at least the inner wall of a hollow tube(or parison P) extends from an extruder on top of the head 11 through aresin inlet 16 a and a resin passageway 16 b and expands annularly intothe die 12. Furthermore, a ring piston (extruding means) 19 adapted toreciprocate down to the first annular passageway 16 is disposed withinthe head 11 and a first molten resin accumulation chamber 20 for poolingmolten resin temporarily in association with the reverse (upward)movement of the ring piston 19, the accumulation chamber 20 communicateswith the first annular passageway 16 and annular slit 15. The firstmolten resin in the first resin accumulation chamber 20 is extrudedcontinuously in response to the forward movement of the ring piston 19.The second annular passageway (intermediate passageway) 17, for thesecond resin such as the hard resin, which forms intermittently orperiodically the intermediate wall of the multilayer segment of thehollow tube and the third passageway (outer annular passageway) 18, forthe third resin forming the outer covering layer, which forms the outerwall of the hollow tube are also supplied with molten resin. The secondmolten resin in a second resin accumulator 21 is extruded intermittentlyor periodically in response to the forward movement of the plunger 23,and the third molten resin in a third resin accumulator 22 is extrudedcontinuously in response to the forward movement of the plunger 24. Thesecond and the third molten resins temporarily pooled in respectiveaccumulators 21, 22 are fed on actuation of corresponding plungers 23,24. In response to the return movement of the plungers 23, 24, eachmolten resin is supplied from the extruder means connected to the head11 and pooled in the accumulators 21, 22. The molten resins flow throughthe first through third annular passageways 16, 17, 18 and thesetributary resin streams converge within the die 12. The outer coveringresin layer may be formed with the soft resin or the hard resin.

The head 11 is further provided with annular partition walls 25 a, 25 b,25 c for isolating the annular passageways 16, 17, 18. In thisembodiment, the supply means for feeding molten resin to the annularpassageways 16, 17, 18 is comprised of the extruder and either the ringpiston or the plungers.

In operation, the rod 13 is driven back (in the upward direction) toengage the lip portion of the die 12 with the mandrel 14 in closecontact and the ring piston 19 is driven back axially to pool the moltenresin for the inner wall in the first resin accumulation chamber 20,while the second and third annular passageways 17, 18 are respectivelysupplied with the second and third molten resins. Then, the rod 13 isdriven forward (downward) to make an annular slit 15 between the lip ofdie 12 and the mandrel 14, the ring piston 19 and plunger 24 arecontinuously driven forward, and the plunger 23 is intermittently drivenforward, whereupon a parison P having a three-walled portion (segment)is formed by convergence of resin streams and extruded from the annularslit 15. After formation of the parison, the rod 13 is driven back(upward) to close the annular slit 15 with the mandrel 14 and the ringpiston 19 is driven back to accumulate molten resin in the firstaccumulation chamber 20, while the plungers 23, 24 are driven back tolet the accumulators 21, 22 accumulate molten resin for the intermittentintermediate wall and the molten resin for the outer wall to be suppliedto the second and third annular passageways 17, 18, respectively. Bycausing the above operation to be repeated, the parison P and hollowtube can be produced on an intermittent basis. However, if, in thisoperation, the rod 13 is driven back after formation of parison P withsection adjustment, a gap will be formed between the die 12 and mandrel14 so that the resin will leak out to cause a loss of material.

Unlike the conventional arrangement where simultaneously with theopening of the annular slit 15 in response to the advancement of rod 13,the die 12 is displaced radially from the original position where it isclosely abutted against the mandrel 14 (that is the reference positionof the die 12 where the axes of mandrel 14 and die 12 are aligned and nogap is formed between the die 12 and mandrel 14) to adjust the thicknessand thereby form a parison P uniform in overall thickness before the rod13 is driven back. Therefore, in the present invention, the die 12 isreset to the original position after formation of parison P and, then,the rod 13 is driven back. In this arrangement (i.e. centeringoperation), the die 12 can be returned to the original position to bringthe axes of die 12 and mandrel 14 into alignment before the rod 13 isdriven back to thereby insure a close contact between the die 12 andmandrel 14, with the result that no gap is formed between them.

Thus, the apparatus of the present invention comprises a thicknessadjusting means for displacing the die 12 from its original position inassociation with the advance of the rod 13 to adjust the thicknessvariation of the parison P extruded from the annular slit 15, the ringpiston 19, and plungers 23, 24 (extruding means) for extruding moltenresins from the resin accumulation chamber 20 and accumulators 21, 22from the annular slit 15 to form a parison P, a resetting means forresetting the die 12 to the original position in response to completionof extrusion of molten resin, and the hydraulic cylinder (rod-drivingmechanism) for driving the rod 13 forward to open the annular slit 15and driving the rod 13 back to close the annular slit 15 in response tothe resetting of the die 12.

The thickness adjusting means and resetting means mentioned above mayeach comprise a biasing means for preenergizing the die 12 inwardly in aplurality of circumferential positions and a pressing means movableforward and backward and adapted to press the die 12 inwardly againstthe biasing means. For example, as shown in FIGS. 3 and 4, there isprovided a biasing member 27 equipped with a spring 26 a for biasing thedie 12 inwardly and a pressor rod 26 b for pressing a flat surfaceformed in the die 12 by means of the preenergizing force of the springin two circumferential positions adjacent to each other 90° apart on thedie 12. Disposed in the position diametrically opposite to the positionpreenergized by the biasing member 27 is a driving mechanism 29comprising a hydraulic cylinder 28 (as a pressor means) operable withrespect to the die 12 and a pressing rod 28 b for transmitting thepressing force of the hydraulic cylinder 28 to the flat surface of thedie 12. Namely, a plurality of the opposite positions of the die arepressed inwardly (or radially) by the biasing means and the pressingmeans. By means of such thickness adjusting means and resetting means,the die 12 is displaced according to the stroke of the cylinder 28 and,as the pressing force of the cylinder 28 is released, the spring 26 ofthe biasing member 27 resets the die 12 rapidly to the original positionprior to thickness adjustment. Furthermore, since the die 12 can bedisplaced in two diagonal directions by the hydraulic cylinders 28 adisposed in diagonal orientations, the variation in thickness can beeffectively precluded even when a plurality of layers are extruded toform a parison P. The displacement and resetting of die 12 can becarried out by means of one set of thickness adjusting means andresetting means, it is more advantageous to displace the die 12 in atleast two positions for a more accurate thickness adjustment.

The bias in thickness of parison P, assuming that the extrusion speed isunchanged, is chiefly associated with the size and thickness of parisonP and a correlation is found between the size and thickness of parison Pand the necessary degree of thickness adjustment (degree of adjustmentof the slit dimension). Therefore, the adjustment of this bias can bemade automatically according to the above-mentioned correlation.

The extruding means for extruding the molten resins from the annularslit 15 are not limited to the ring piston 19 and plungers 23, 24 but avariety of other extrusion mechanisms for molten resins can be employed.In other to preclude bending and insure a precision forming of parisonP, the extruding means is preferably the ring piston 19. For theextrusion of the second and third resins, too, ring pistons 19 can beused in lieu of the accumulators and plungers 23, 24. The third annularpassageway 17, the accumulator 22 and the plunger 24 are not essentialfor the present invention.

The operation described above can be performed by means of an automaticsystem comprising an input means with which the desired values ofparison P such as the total size, total wall thickness, and thethickness and length of the second resin layer can be entered, anoperation circuit which calculates the amount of control necessary foreach action according to the input data and the data (the thicknessadjustment data corresponding to the factors of parison P such as thetotal size, total thickness, and the thickness and length of the secondresin layer) stored in a memory (memory means), and a control circuitwhich outputs a control signal representing the amount of control to beapplied to a driving circuit for the mechanism. Regarding the action forthe thickness adjustment (step 1), for instance, the size and wallthickness of parison P are entered from the input means of the automaticsystem and based on the data (thickness adjustment data correlated withthe size and wall thickness of parison P) stored in the memory (memorymeans), the stroke (control amount) of the hydraulic cylinder as thethickness adjusting means or the resetting means is computed in theoperation circuit. In the above control system, the control circuitresponding to a start signal from the input means outputs a forwardsignal to the driving circuit for the hydraulic cylinder to drive therod 13 and, at the same time, the control circuit outputs a drivingsignal corresponding to the amount of control to the driving circuit forthe hydraulic cylinder 28 a for thickness adjustment to drive thehydraulic cylinder by a given distance either way.

Furthermore, as mentioned above, the control circuit responding to thestart signal outputs the control amount data calculated by the operationcircuit according to input thickness data as a forward signal to thedriving circuit for the hydraulic cylinder for driving the rod 13 tocause the rod 13 to slide forward by a distance corresponding to thewall thickness of parison P (step 2).

With regard to the extruding action (step 3) for forming the parison P,the control circuit responding to the start signal or sensing completionof extension of the hydraulic cylinder 28 a for thickness adjustmentoutputs a forward signal to the driving circuit for the hydrauliccylinder which drives the ring piston 19 and the driving circuit for thehydraulic cylinders which drives the plungers 23, 24 to thereby causethe ring piston 19 and plungers 23, 24 to slide each by a givendistance.

In the extruding step, the second molten resin is intermittently orperiodically extruded corresponding to the thickness and length of thesecond resin layer of the multilayer segment by the plunger 23 withcontrolling the sliding speed of the plunger 23, and the third moltenresin continuously extruded to form the outer covering layer.

The resetting action (step 4) takes place as follows. In response tocompletion of movement of the ring piston 19 and plungers 23, 24, thecontrol circuit outputs a reset signal to the driving circuit of thehydraulic cylinder 28 for thickness adjustment to cause an actionopposite to the thickness adjustment action so as to reset the hydrauliccylinder 28 a to the original position. The control circuit respondingto the reset signal or completion of the resetting motion of thehydraulic cylinder outputs a back signal to the driving circuit of thehydraulic cylinder for driving the rod 13 to thereby drive the rod 13 ina reverse direction to close the annular slit 15 (step 5).

Furthermore, in response to a signal representing completion of thereverse movement of rod 13, the control circuit outputs a back signal tothe driving circuit of ring piston 19 and the driving circuit ofplungers 23, 24 to cause the ring piston 19 and plungers 23, 24 toretreat each by a predetermined distance and thereby letting apredetermined amount of molten resin accumulate in each of the moltenresin accumulation chamber 20 and accumulators 21, 22 (step 6).

Then, in response to a signal representing completion of the reversemovement of the ring piston 19 and plungers 23, 24, the control circuitoutputs a forward signal to the driving circuit for the rod 13 and thedriving circuit for the hydraulic cylinder 28 a for thickness adjustmentto repeat the thickness adjustment action (the action of step 1) throughthe action of step 16 (resin accumulation).

With the extrusion molding apparatus described above, the step ofadvancing the rod 13 to open the annular slit 15, the thicknessadjusting step for displacing the die 12 from its original position, theextruding step of extruding the resins in the molten resin accumulationchamber 20 and the accumulator 21, 22 from the annular slit 15 to form aparison P, the step of resetting the die 12 to the original position,the step of driving the rod 13 in a reverse direction to close theannular slit 15, and the step of accumulating molten resins in themolten resin accumulation chamber 20 and accumulators 21, 22 can beexecuted in repetition for continuous production of hollow tubes at apredetermined interval. Particularly since the die 12 is reset intoaxial alignment with the mandrel 14, i.e. centering of the die 12 andthe mandrel 14, before the rod 13 is driven back, the annular slit 15can be tightly closed with the mandrel 14 even after thicknessadjustment, the creation of a gap between die 12 and mandrel 14 isprecluded. Therefore, a parison of uniform wall thickness can beproduced and molten resin leaks can be prevented even when thicknessadjustment is made.

Although the above description pertains to the production of a parisonhaving a three-walled structure, the multilayer segment of the parisonmay be a two-walled (layered) structure comprising the soft resin layerand the hard resin layer.

The extrusion molding apparatus of the present invention is also of useas a preblowing means which infuses or supplies a gas such as air intothe parison for shape retention through the bore formed axially in therod 13.

In the above apparatus, the pressing means constituting the thicknessadjusting means and resetting means are not restricted to hydrauliccylinders 28 but can be other known reciprocal driving means capable offorward and reverse actions and having a positioning function, such asreciprocal mechanisms utilizing a stepping motor (e.g. a worm and racksystem which transforms the rotary motion of a stepping motor into areciprocating motion through gearing), for instance. The biasing meansmay be any known means, for example a variety of cylinders, shockabsorbers, and other means capable of biasing with a constant pressure.

In the production apparatus and molding method of the present invention,the sequence is that a parison is formed after thickness adjustment andthe rod is driven back after the die has been reset to the originalposition (centering position). Therefore, not only the sectionaluniformity of the parison can be insured but also the slit between thedie and mandrel can be hermetically closed. Therefore, resin leaks canbe positively prevented to permit effective utilization of the resin forthe formation of the parison and hollow tube. Furthermore, sincethickness adjustment is made in a plurality of positions of the die, thevariation in wall thickness can be adjusted with high accuracy even inthe production of a tube having a multi-layer portion so that parisonsand hollow tubes of exact thickness can be manufactured.

What is claimed is:
 1. A blow-molded hose of a synthetic resin, whichcomprises a monolayer segment formed comprising a first resin layer ofeither a soft resin or a hard resin and a multilayer segment formed withthe first resin layer and a second resin layer of a hard resin when thefirst resin layer is a soft resin or a soft resin when the first resinlayer is a hard resin, and the thickness ratio, in the multilayersegment, of the first resin layer is thinner than that of the secondresin layer, wherein said soft resin comprises at least one memberselected from the group consisting of: a low-density polyethylene, amiddle-density polyethylene, a linear low-density polyethylene, anethylene copolymer, a plastisized polyvinyl chloride, a polyurethane,and a thermoplastic elastomer, said hard resin comprises at least onemember selected from the group consisting of: a polypropylene, ahigh-density polyethylene, an ethylene-propylene copolymer, and apolyvinyl chloride, said monolayer segment and said multilayer segmentalternate in the longitudinal direction of the hose, and the thicknessof the hose is constant along the longitudinal direction of the hose. 2.A blow-molded hose of a synthetic resin as claimed in claim 1, whereinsaid monolayer segment is formed with the soft resin and forms aconnection end at the edge of the hose.
 3. A blow-molded hose of asynthetic resin according to claim 1, wherein the thickness ratio ofsaid first resin layer of the soft resin relative to said second resinlayer of the hard resin in said multilayer segment is 5/95 to 95/5.
 4. Ablow-molded hose of a synthetic resin according to claim 1, wherein thethickness of said second resin layer of the hard resin is thicker thanthat of said first resin layer of the soft resin in said multilayersegment.
 5. A blow-molded hose of a synthetic resin according to claim1, wherein said monolayer segment is formed with the soft resin.
 6. Ablow-molded hose of a synthetic resin according to claim 1, whichcomprises a monolayer segment of a soft resin and a multilayer segmentcomprising an inner soft resin layer, an intermittent intermediate hardresin layer and an outer covering layer.